There has been a limit in improving the recording density of a magneto-optical recording medium by being dependent on the size of a light spot of a light beam used for recording and reproducing on and from the recording medium. This is because a diameter of the light spot on the recording medium becomes a diameter of a recorded bit. However, recently, a magneto-optical recording medium has been proposed wherein recorded bits with a size smaller than the size of a light spot can be reproduced.
Normally, the light beam for use in optical recording is converged to a diffraction limit by a converging lens. Therefore, the light intensity distribution shows a Gaussian distribution, and thus the temperature distribution due to the light beam on the recording medium also exhibits the Gaussian distribution. As a result, a spot having a temperature above a predetermined temperature becomes smaller in size than the size of the light spot. Consequently, a significant improvement in the recording density can be achieved if only the spot having a temperature above the predetermined temperature is used for reproduction.
Referring to FIG. 16, the following description will discuss a magneto-optical disk wherein a recorded bit with a size smaller than the size of a light spot can be reproduced.
The magneto-optical disk is mainly consisted of a substrate 21 having a readout layer 23 and a recording layer 24 formed on a surface thereof. The recording layer 24 has great coercive force at room temperature. On the other hand, the readout layer 23 has small coercive force at room temperature. When the temperature of an area of the readout layer 23 to be reproduced is raised by irradiating thereon with a reproduction-use light beam, the magnetization direction thereof becomes coincident with the magnetization direction of the recording layer 24 due to the effect of the recording layer 24. That is, the magnetization of the recording layer 24 is copied to the readout layer 23 by exchange coupling force between the readout layer 23 and the recording layer 24.
Recording on the described magneto-optical disk is executed by the ordinary thermomagnetic writing method. When the recorded bits are to be reproduced, it is necessary to initialize the magnetization direction of the readout layer 23 so as to make it coincident with the predetermined direction (upward in the figure) by applying an external magnetic field for initializing from a magnetic field generating device 26. Then, by projecting thereto a reproduction-use light beam 27, the temperature of the readout layer 23 is locally raised. As a result, the portion having a temperature rise of the readout layer 23 has small coercive force, and the magnetization direction of the recording layer 24 is copied to the readout layer 23 by the exchange coupling force. In this way, since only the information stored in the center area which has received the reproduction-use light beam 27 and undergone a temperature rise is reproduced, recorded bits with a size smaller than that of the light spot are permitted to be read out.
However, when using the discussed magneto-optical disk, the following problem arises. During reproduction, a recorded bit that has been copied to the readout layer 23 from the recording layer 24 remains as it is even after the temperature of the spot has cooled off. This means that when a spot to be irradiated by the light beam 27 is shifted by a rotation of the magneto-optical disk so as to reproduce the next bit, the bit previously copied still exists within the light beam 27 and tends to be reproduced. This causes noise and has prevented improvement in recording density.
Furthermore, the magneto-optical disk having the described configuration cannot be provided with the overwriting function through a light intensity modulation method. This presents another problem by requiring a long time for data writing.